In known very low intermediate frequency (VLIF) receivers and direct conversion receivers (DCRs), the mixer injection frequency is relatively near or at the desired signal frequency. In VLIF receivers for example the mixer injection frequency is equal to the receive signal frequency plus or minus the intermediate frequency. In direct conversion receivers, the injection frequency is equal to the signal frequency.
The proximity of the mixer injection frequency and the receive signal frequency in VLIF and direct conversion receivers results in the mixer injection frequency being within the passband of the pre-selection filter, usually before the low noise amplifier (LNA) portion of the receiver. Prior art FIG. 1 illustrates a frequency spectrum of a conventional VLIF receiver system with chopping and mixing products, where the mixer injection frequency and the receive signal frequency are within the passband of the pre-selection filter. After mixing, the conventional chopper output product, which may be an intermediate or baseband signal, is within the VLIF Passband or direct conversion receiver passband.
In prior art FIG. 1, in conventional VLIF and direct conversion receivers, the pre-selection filter does not prevent leakage of the mixer injection frequency back through the antenna, since the mixer injection frequency is within the receive passband 110. Mixer injection frequency leakage is undesirable because it causes interference. Communication specification standards thus impose limits on this leakage.
The proximity of the mixer injection and receive signal frequencies in VLIF and direct conversion receivers also tends to aggravate the coupling of harmonics to the voltage controlled oscillator (VCO), thus degrading receiver performance. Harmonic or fundamental frequency coupling to the VCO is especially troublesome at higher signal strengths and in applications where the VCO is integrated on the same integrated circuit (IC) as the receiver.
The various aspects, features and advantages of the present invention will become more fully apparent to those having ordinary skill in the art upon careful consideration of the following Detailed Description of the Invention with the accompanying drawings described below.